The present invention relates in a general way to headlamps of the elliptical type for motor vehicles.
An elliptical headlamp conventionally comprises a light source such as an incandescent filament or the luminescent arc of a discharge lamp, this source being placed in a first focal region of a mirror so that the light reflected by it is directed towards a second focal region situated in front of the first one. A lens, generally plano-convex, is focused on this second focal region, so as to project the light spot formed in said second focal region onto the road.
This light spot can be modeled, for example with a mask, to form a beam with cut-off as required, such as a dipped beam, an upper edge of this mask defining the profile of this cut-off.
Because of this possibility of forming a sharp cut-off, and because of the excellent recovery by the mirror of the light flux emitted by the source, such headlamps have been used successfully for many years to form dipped European beams with cut-off in a xe2x80x9cVxe2x80x9d shape.
In contrast, to produce a dipped beam in accordance with the standards in force in the United States of America, relying on a headlamp of the elliptical type poses more difficulty.
This is because one of the specific features of these standards lies in the requirement for a maximum brightness in the axis of the road which is substantially twice as high as in the European standard, whereas the standardized lamps used in the USA (for example of the 9006 standardized type) have a lower brightness (for identical overall flux and diameter) than the European lamps (for example of the standardized H7 or H9 type) since their filament is substantially lengthy.
Under these conditions, it can be understood that a headlamp possessing a conventional mirror in the shape of an axisymmetric ellipsoid will, in the absence of a mask, generate a beam exhibiting, in the vicinity of the optical axis, a central part of relatively great thickness, due to the images of the filament, which are elongated in the vertical direction and which are produced by the areas of the mirror which are situated immediately above and below the lamp, whereas regions of the mirror laterally very far from the lamp will generate lateral parts of the beam, corresponding to small, horizontally elongated, images of the filament, which will exhibit a substantially reduced thickness by comparison with said central part.
It is conventionally sought, however, in order to produce a satisfactory dipped beam, to give the beam, below the cut-off, a thickness which remains substantial over a significant extent in width, this being done particularly so as properly to illuminate the verges of the road.
It is also sought to obtain this result without degrading the concentration spot of the beam, which is directed in the axis of the road or slightly to the right (in the case of traffic driving on the right).
An object of the present invention is to remedy these limitations of the state of the art, and to generate, with the aid of a specifically designed mirror, a beam which, once partially shaded by a cut-off mask in a way which is known in itself, gives particularly satisfactory illumination.
Hence the present invention relates to a motor vehicle headlamp, comprising a light source, a mirror possessing first and second focal regions, and a converging lens, the source being placed in the first focal region and the lens possessing a focus situated in the second focal region, the mirror and the lens having axes which are essentially coincident defining an optical axis of the headlamp, and the headlamp further including a mask placed in the region of the focus of the lens, in order thus to project a beam an upper cut-off of which is defined by said mask, and the mirror being able to concentrate the light, in the vertical direction, towards a vertical focusing baseline extending substantially horizontally and transversely to the optical axis and passing close to the focus of the lens, wherein the mirror possesses at least one corrected vertical focusing area able to concentrate the light, in the vertical direction, towards vertical focusing locuses remote from said vertical focusing line in the axial direction, in order thus to increase the thickness of the light reflected by said area.
Preferred, but not limiting, aspects of the headlamp according to the invention are as follows:
two corrected focusing areas are provided, situated on either side of an axial vertical plane.
said areas are extreme lateral areas of the mirror.
the or each corrected vertical focusing area possesses a corrected vertical focusing line separate, in the axial direction, from said vertical focusing baseline.
the mirror further comprises at least one area for vertical offsetting of the light able to generate a radiation which, on average, is offset upwards or downwards with respect to the vertical focusing baseline.
the headlamp comprises a vertical offsetting area situated in the central region of the mirror and able to offset the light upwards, and two vertical offsetting areas situated on either side of said central region and able to offset the light downwards.
the or each vertical offsetting area is constructed from sections of axisymmetric ellipsoids a first focus of which is situated above or below the source and a second focus of which is situated on a vertical focusing line associated with said area.
the first focus of each ellipsoidal section is situated substantially in the vertical to the center of the source.
the vertical distances between the first focuses of the various ellipsoidal sections and the center of the source vary progressively from one section to the other.
from the back of the mirror to its lateral edges, said vertical distances pass progressively from a first value corresponding to a first focal position situated below the source to a second value, of opposite sign, corresponding to a first focal position situated above the source.
in the region of the lateral edges of the mirror, said vertical distance is essentially zero.